The present invention relates to a modified epoxy resin composition and a process for modifying an epoxy resin which enhances the overall properties of the resin composition without deteriorating the original excellent properties thereof.
Epoxy resins have usually excellent properties such as excellent mechanical strength, adhesion, heat resistance and chemical resistance, and hence, they are widely used for coating compositions, electrical insulators, materials for civil engineering and construction, adhesives, composite materials, or the like. On the other hand, the cured epoxy resin products are usually inferior in flexibility, and hence, attempts have been made to improve the fragility thereof and to give thereto excellent impact resistance and thermal shock resistance. For instance, it has been proposed to modify the epoxy resin by adding thereto a modifying agent such as polysulfide, polychloroprene, polybutadiene, dimeric acid, polyester ether, polyamide, and so forth, but such methods have some disadvantages in that these result in deterioration of the original properties of the epoxy resin such as the mechanical strength, electric properties, heat resistance, and solvent resistance, corrosion resistance, and further the most suitable method must be individually selected in accordance with the intended utilities. In the case of employing the resin for a coating composition, the epoxy resin should have good impact resistance and adhesion to the substrate as well as solvent resistance, corrosion resistance, and further other properties such as compatibility of the epoxy resin with the modifying agent, homogeneity and film-forming properties of the vehicle resin, crosslinking curing properties, or the like. Such requirements have never been satisfied by the known techniques.
Recently, epoxy resins have been used as a material for cathode electrocoating compositions suitable for coating automobiles, and for such a purpose, there have been proposed various methods for improving the properties of the epoxy resins in order to produce a coating composition having excellent chipping resistance, corrosion resistance, and so forth. Among these, it is proposed to use as a flexibilizer a liquid polybutadiene which has been used for the preparation of anode electrocoating compositions. However, the liquid polybutadiene has insufficient compatibility with the epoxy resin, and even if it is chemically bonded with the epoxy resin by means of an appropriate reactive group, it is still difficult to obtain an excellent composition which can give a clear uniform coating layer. In order to eliminate such a drawback, particularly to improve the compatibility of the flexibilizer with the epoxy resin, various methods have been proposed. For example, it is disclosed in Japanese Patent Publication (unexamined) No. 97632/1979 that a base resin for a cationic electrocoating composition is obtained by reacting a butadiene-acrylonitrile copolymer containing carboxyl groups at both terminal thereof with an epoxy resin. According to this method, the compatibility with the epoxy resin is fairly improved owing to the acrylonitrile group, but it is still insufficient in the clarity of film and flexibility. Moreover, since this copolymer is a comparatively high molecular weight functional polymer, use thereof is restricted; for example, it is required to specify the ratio of the copolymer to the epoxy resin in order to give the desired homogeneity of the electrocoating composition or to regulate the concentration of a cationic group in order to give the desired electrophoresis. This method is also disadvantageous in view of the high cost.
Another approach for improving the flexibility by using a liquid polybutadiene is disclosed in Japanese Patent Publication (unexamined) No. 5933/1980, wherein a reaction product of an epoxy resin with a conjugated diene polymer containing a carboxyl group at the terminal or intermediate of the molecule is used as a base resin for a cationic electrocoating composition. The liquid polybutadiene used in this method is substantially a liquid polybutadiene having a high 1,2-vinyl structure and having carboxyl groups at both terminals, and the liquid polybutadiene is used for improving smoothness and curing properties of the coating film. These liquid polybutadienes having high a 1,2-vinyl structure consist mainly of butadiene skeleton and are still inferior in the compatibility with the epoxy resins.
Another method is proposed in Japanese Patent Publication (unexamined) No. 5932/1980. This literature discloses a resin composition suitable for a cathode precipitating type electrocoating which comprises predominantly a reaction product of a basic resin and an amine-modified epoxy resin, wherein the basic resin is obtained by reacting an anhydrous maleic acid adduct of a liquid polybutadiene or a natural drying oil with a diamine containing a tertiary amino group within the molecule (e.g. N,N-dimethylaminopropanediamine) and an aminoalcohol (e.g. monoethanolamine) or a diamine containing a hydroxy group (e.g. 2-(N,N-dihydroxymethylamino)ethylamine), by which the liquid polybutadiene or natural drying oil is bonded with a tertiary amino group and hydroxy group via an imidohydrocarbon bond. Since the basic resin used in this method contains a large amount of an aliphatic tertiary amino group having a comparatively strong basicity and has a polymerization catalytic activity to the epoxy group, when the basic resin is reacted with the epoxy resin in a system containing an excess amount of epoxy groups, a polymerization reaction due to self ring-opening of the epoxy groups occurs such that the esterification reaction between the carboxyl group of the basic resin and the epoxy group of the epoxy resin is insufficient, which unfavorably results in gelation of the system. Accordingly, in the method of Japanese Patent Publication (unexamined) No. 5932/1980, the amination of the epoxy resin is carried out while preventing the above-mentioned undesirable side reaction by means of carrying out the reaction in the presence of an equimolar amount of an amine to the excess epoxy groups, or charging the basic resin into the system after partial progression of the amination reaction.
Thus, it is required to introduce flexibility to epoxy resins not only in the field of cationic electrocoating but also in other coating fields and in the production of various molded products.
Under the circumstances, the present inventors have intensively studied improving a process for modifying epoxy resins in order to obtain an improved modified epoxy resin having excellent properties suitable for various requirements and various utilities. As a result, it has been found that the desired modified epoxy resin can be obtained by reacting or mixing an epoxy resin with a specified modified adduct of a conjugated diene polymer or copolymer.